Terahertz phase recovery imaging system and imaging method based on unequal spacing measurement

Abstract

The present invention relates to a terahertz phase recovery imaging system and imaging method based on unequal spacing measurement, and its purpose is to solve the problem that the terahertz detector can only record the amplitude information of the complex-valued image but lacks phase information and improve the imaging quality. Specifically, it includes: first, using a lensless terahertz imaging system to translate the detector in the direction of the optical axis to realize the collection of diffraction pattern amplitude values ​​of multiple recording distances, and multiple recording distances adopt nonlinear interval measurement; second, iterative phase recovery imaging: by performing a diffraction propagation iterative algorithm on the complex amplitude of the recorded image in sequence between each recording surface, finally obtaining a high-quality object surface complex amplitude distribution image. The present invention does not require any complex optical imaging devices, and can obtain high-quality object surface complex amplitude images through very few measurements and fast data processing. The overall system is simple and compact, which can greatly expand the application field of terahertz imaging.

Description

Technical Field

[0001] The present invention belongs to the technical field of terahertz imaging, and aims to solve the problem that a terahertz detector can only record amplitude information of a complex-valued image but lacks phase information, and to improve imaging quality. Background Art

[0002] The frequency of terahertz waves ranges from 0.1-10THz, between the infrared spectrum and the microwave spectrum. Its non-destructive and non-ionizing characteristics have enabled applications in biomedicine, safety testing, and quality control. The terahertz waves used in these methods are mostly based on terahertz radiation generated by femtosecond laser pulses. Their system structure is complex and lacks high radiation intensity. These shortcomings limit the widespread application of terahertz imaging. Continuous (CW) terahertz waves generated by electronic devices are simple, compact, and cost-effective, becoming a new approach to terahertz imaging technology. However, most continuous terahertz imaging systems can only measure intensity values ​​without phase information, so various phase recovery methods that obtain the complex amplitude distribution of the object surface through iterative calculations have been proposed.

[0003] In recent years, the iterative phase recovery algorithm based on the Gerchberg-Saxton (GS) algorithm has been applied to terahertz phase imaging. This algorithm obtains the complex amplitude information of the object plane by repeatedly propagating and inverting the light field information between the recording plane and the object plane. However, this algorithm requires some prior information of the object as the constraint condition of the iteration and the convergence speed of the algorithm is slow. The phase recovery method based on multi-recording equal-interval measurement has been proposed and has been promoted and applied in the terahertz field. Since this iterative algorithm does not require prior knowledge of the object plane, it can obtain accurate complex amplitude images of the object plane. However, since this method uses equal-interval measurement, it is necessary to collect multiple recording planes. Under the condition that the existing terahertz detection equipment is not mature, it takes a lot of testing time. At the same time, in the process of iterative calculation, the use of more recording planes will cause corresponding noise accumulation, which seriously affects the quality of the complex amplitude image finally recovered. Therefore, at this stage, there is an urgent need for a fast terahertz phase recovery imaging system that can well suppress the influence of noise. Summary of the invention

[0004] In view of the above technical deficiencies, the present invention provides a lensless terahertz phase recovery imaging system based on unequal spacing measurement and a phase recovery imaging method based on amplitude measurement. The system adopts a lensless terahertz wave measurement system, and only requires a terahertz emitter and detector to complete the acquisition of object surface information. Through an iterative recovery algorithm based on Rayleigh-Sommerfeld diffraction, it recovers high-quality complex amplitude images without the constraint of prior information on the object surface when using less measurement data.

[0005] The technical solution adopted by the present invention to solve the technical problem is: a terahertz phase recovery imaging method based on unequal spacing measurement, which is used to simultaneously recover the terahertz image of the imaging object in terms of amplitude and phase parameters, comprising the following steps:

[0006] S1. Set a transmitting source at the origin of the three-dimensional coordinate system, take the terahertz wave transmitting direction as the z-axis, and sequentially set the imaging object and the detector along the z-axis direction; control the terahertz wave transmitting source to irradiate the imaging object;

[0007] S2, setting iteration rounds with unequal sampling intervals, and executing a single iteration step in each iteration process, and ending the iteration round according to the iteration end criterion;

[0008] The single iteration step includes:

[0009] i. Using the unequal sampling spacing sequence {Z} as the adjustment parameter, the terahertz detector on the three-dimensional displacement stage is driven from the initial sampling spacing position to the farthest sampling spacing position, and is located at each sampling position of the unequal sampling spacing on the z-axis, gradually moving away from the emission source in the positive direction and then gradually approaching in the negative direction; at each sampling position, the terahertz detector is controlled to move in the xy two-dimensional plane at the position according to the preset step length and trajectory, and the imaging image intensity at the current position is scanned and collected; a series of intensity sequence images {I} of the imaging object transmitted by the terahertz wave are obtained;

[0010] ii. For a series of intensity sequence images, the phase complex amplitude image with unequal spacing measurements based on the Rayleigh-Sommerfeld diffraction propagation method is used to iteratively reconstruct the image, so as to achieve simultaneous recovery and reconstruction of the two parameters of amplitude and phase.

[0011] In a single iteration step:

[0012] The sampling interval sequence from the initial sampling interval position to the farthest sampling interval position is {Z}={Z0, ΔZ i-1}; where Z0 is the initial sampling interval and is set to a fixed value, ΔZ i-1 is a natural number of unequal sampling intervals, i=2…n+1;

[0013] The intensity sequence image {I} is a sequence image of intensity values ​​at sampling points corresponding to each sampling interval, {I} = {I0, I i-1}, where I0 is the intensity value of the initial sampling interval point, I i-1 is the measured intensity image of points with unequal sampling intervals, i=2...n+1 is a natural number.

[0014] Unequal sampling interval ΔZ i-1 The setting is based on the Fresnel number difference ΔF between adjacent diffraction recording patterns. # value to set.

[0015] The above ii. for a series of intensity sequence images, the Rayleigh-Sommerfeld diffraction propagation method is used to reconstruct the complex amplitude images measured at unequal intervals, including:

[0016] a). At the initial sampling interval Z0, the initial phase value φ(0) is randomly selected, then the complex amplitude of the initial object surface is

[0017] b) Starting from n=2, use the Rayleigh-Sommerfeld diffraction formula to sequentially sample U0 at different sampling intervals ΔZ i-1 The forward diffraction propagates to the next acquisition position, and then travels back to the position at the initial sampling interval Z0 to obtain the complex amplitude image of the first position;

[0018] c) Use the Rayleigh-Sommerfeld formula to reversely diffract the complex amplitude image of the first position after the iteration is completed. The propagation distance Z0 is obtained to obtain a clear complex amplitude image of the object under test in this cyclic iteration of unequal sampling intervals.

[0019] In the forward propagation and reverse return process in step b), the measured intensity values ​​I at each unequal sampling interval position are i-1 Update the amplitude by the square root of , and keep the phase value calculated in the previous step unchanged, so as to obtain the new U in (x, y) is used as the complex amplitude of the input plane for the next step of propagation calculation, and the propagation of the phase complex amplitude image at unequally spaced positions continues.

[0020] The Rayleigh-Sommerfeld diffraction propagation formula is:

[0021]

[0022] Where λ is the incident terahertz wavelength, k = 2π / λ is the wave number, j represents the imaginary part of the complex number, z is the distance between adjacent recording surfaces, and U in is the complex amplitude in the input plane, U out is the complex amplitude in the output plane, (x,y) and (f x ,f y ) are spatial domain and frequency domain coordinates, F and F -1 are the forward and inverse Fourier transforms.

[0023] The iteration rounds of the unequal sampling intervals are terminated according to the following iteration termination criteria:

[0024]

[0025] In each round, the root mean square error between the intensity image of the complex amplitude restored in each iteration and the intensity recording surface at the first Z0 is calculated: , where t is the iteration round and M is the number of N pixels;

[0026] When the MSE is lower than the set threshold, the iteration is terminated, and the iterative process of phase recovery for round i is completed.

[0027] A terahertz phase recovery imaging system based on unequal spacing measurement, comprising: a terahertz emission source, an imaging object, a terahertz detector electronic computer and a three-dimensional displacement stage;

[0028] The emission source is arranged at the origin of the three-dimensional coordinate system, and the image object and the detector are arranged in sequence in the direction of the z-axis with the terahertz wave emission direction as the z-axis; the emission source emits terahertz waves, and the terahertz detector obtains the intensity value of the imaged object transmitted by the terahertz waves;

[0029] The three-dimensional displacement stage includes a servo motor, a driver, an x-axis transverse linear motion module, a y-axis longitudinal linear motion module, and a z-axis terahertz wave emission direction motion module. The terahertz detector is fixed on the transverse motion module. The servo motor drives the transverse linear motion module and the longitudinal linear motion module to move, thereby adjusting the position of the terahertz detector in the xy two-dimensional plane. The servo motor drives the z-axis terahertz wave emission direction motion module to change the sampling positions of the various unequal sampling intervals residing on the z-axis. The driver is connected to an electronic computer via a communication interface.

[0030] The electronic computer internally comprises a storage unit and a processing unit. The storage unit stores a program, and the processing unit loads the program to execute the steps of the method described above.

[0031] The servo motor is used to drive the z-axis terahertz wave emission direction motion module to change the sampling positions of the unequal sampling intervals on the z-axis, including: using the unequal sampling interval sequence {Z} as an adjustment parameter, driving the terahertz detector on the three-dimensional displacement stage from the initial sampling interval position to the farthest sampling interval position, and residing at the sampling positions of the unequal sampling intervals on the z-axis in a forward direction away from the emission source and then in a reverse direction asymptotically approaching.

[0032] The present invention has the following beneficial effects and advantages:

[0033] 1. The present invention adopts a lens-free terahertz imaging system, which has a simple and compact overall structure, low price and wide application range;

[0034] 2. The present invention adopts nonlinear unequal spacing measurement based on Fresnel number, fully utilizes the high and low frequency information of the diffraction image, reduces the number of recording surfaces required in the iterative algorithm, and greatly saves measurement time;

[0035] 3. The present invention adopts an iterative phase recovery algorithm based on Rayleigh-Sommerfeld diffraction propagation, which improves the convergence speed of the algorithm, better suppresses noise, and restores a high-quality complex amplitude image of the object surface. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is a schematic diagram of a terahertz phase recovery imaging system for unequal spacing measurements;

[0037] Figure 2 It is a schematic diagram of nonlinear unequal spacing measurement based on Fresnel number;

[0038] Figure 3 This is a comparison diagram of the lensless terahertz phase recovery imaging of the present invention using unequal spacing measurements and the imaging recovery using the traditional equal spacing method; DETAILED DESCRIPTION

[0039] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific implementation method of the present invention is described in detail below in conjunction with the accompanying drawings. In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without violating the connotation of the invention, so the present invention is not limited by the specific implementation disclosed below.

[0040] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used in the specification of the invention herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention.

[0041] like Figure 1 The figure shows a schematic diagram of a terahertz phase recovery imaging system based on unequal spacing measurement of the present invention, comprising: an emission source 1 for generating continuous terahertz waves, an imaging object 2, a terahertz detector 3, a high-precision three-dimensional electric translation stage 4, and a computer 5, wherein the terahertz detector and the three-dimensional mobile platform are respectively connected to the computer.

[0042] The terahertz beam emitted from the terahertz emission source is scattered in space and then irradiated on the imaging object. Due to the diffraction effect, a diffraction light field is formed behind the imaging object. The terahertz detector installed on the three-dimensional mobile platform scans and records the diffraction light field intensity at different distances, and transmits the detection value to the computer.

[0043] Figure 2 The figure shows a schematic diagram of nonlinear unequal spacing measurement based on the Fresnel number. In the traditional multi-distance phase recovery test, the measurement of the diffracted light field is carried out at equal intervals. In this method, the Fresnel number that describes the distribution of the diffracted light field is introduced to define the nonlinear unequal spacing. # Defined as

[0044]

[0045] Where D is the longitudinal length of the diffraction pattern and λ is the wavelength of the incident terahertz wave. # value, then the measured distance Δz between adjacent diffraction recording surfaces i This results in nonlinear changes.

[0046]

[0047] where Δz i , Δz i+1 The adjacent unequal spacing.

[0048] The specific phase recovery algorithm is as follows:

[0049] 1. Place the imaging object in the terahertz phase recovery imaging system with unequal spacing measurement, record the initial distance between the imaging object and the terahertz detector mounted on the three-dimensional translation stage as Z0, and collect the intensity image at the initial state as I0. Then move the electric translation stage in the direction away from the object, and select ΔF # =8, and record the intensity images at different ΔZ distances in sequence.

[0050] 2. Randomly select the initial phase value φ(0), then the complex amplitude of the initial object surface is Use the Rayleigh-Sommerfeld diffraction formula to propagate the diffraction of U0 to a distance ΔZ1. The propagation formula is: Where λ is the incident terahertz wavelength, k = 2π / λ is the wave number, j represents the imaginary part of the complex number, z is the distance between adjacent recording surfaces, and U in is the complex amplitude in the input plane, U out is the complex amplitude in the output plane, (x,y) and (f x ,f y ) are spatial domain and frequency domain coordinates, F and F -1 are the forward and inverse Fourier transforms.

[0051] 3. When the next surface propagation is performed, the square root of the measured intensity value is used to replace the calculated amplitude, while keeping the calculated phase value unchanged, and the propagation is performed on n recording surfaces at a time.

[0052] 4. After the recording surface propagates from the first position to the last recording surface along different distances, it then propagates back to the first position by diffraction in the reverse direction, completing one iteration. The root mean square error based on the first intensity recording surface is used as the criterion for the end of the iteration. The root mean square error is defined as: Where t is the iteration round, M is the number of pixels in N. When the MSE is lower than the set threshold, the iterative process of phase recovery is completed.

[0053] 5. Use the Rayleigh-Sommerfeld formula to reversely diffract the complex amplitude image after the iteration to propagate the distance Z0, and finally obtain a clear complex amplitude image of the object under test.

[0054] Example verification:

[0055] In order to compare and verify the effectiveness and superiority of the lensless terahertz phase recovery imaging system based on unequal spacing measurement of the present invention, we used the terahertz phase recovery imaging system of the present invention to perform terahertz phase recovery imaging on a metal paper clip. Figure 3 The first row is the amplitude and phase diagram restored by the traditional equidistant measurement method, and the second row is the amplitude and phase diagram restored by the method of the present invention. The traditional equidistant measurement phase recovery method used 500 iterations to reach convergence, while the unequal spacing measurement phase recovery method of the present invention reached convergence in only 200 iterations. In addition, from the perspective of the quality of the restored image, the image restored by the unequal spacing measurement phase recovery imaging method of the present invention has a clearer outline and better noise suppression, which verifies the advantages of the imaging method of the present invention in terms of speed and high quality.

[0056] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any simple modification, change and equivalent structural change made to the above embodiment based on the technical essence of the present invention still falls within the protection scope of the technical solution of the present invention.

Claims

1. A terahertz phase recovery imaging method based on unequal spacing measurement, used to simultaneously recover the terahertz image of an imaging object in terms of amplitude and phase parameters, characterized in that: The following steps are involved: S1. Set a transmitting source at the origin of the three-dimensional coordinate system, take the terahertz wave transmitting direction as the z-axis, and sequentially set the imaging object and the detector along the z-axis direction; control the terahertz wave transmitting source to irradiate the imaging object; S2, setting iteration rounds with unequal sampling intervals, and executing a single iteration step in each iteration process, and ending the iteration round according to the iteration end criterion; The single iteration step includes: i. Using the unequal sampling spacing sequence {Z} as the adjustment parameter, the terahertz detector on the three-dimensional displacement stage is driven from the initial sampling spacing position to the farthest sampling spacing position, and is located at each sampling position of unequal sampling spacing on the z-axis, gradually moving away from the emission source in the positive direction and then gradually approaching in the negative direction; at each sampling position, the terahertz detector is controlled to move in the xy two-dimensional plane at the position according to the preset step length and trajectory, and the imaging image intensity at the current position is scanned and collected; a series of intensity sequence images {I} of the imaging object transmitted by the terahertz wave are obtained; ii. For a series of intensity sequence images, the phase complex amplitude image with unequal spacing measurements based on the Rayleigh-Sommerfeld diffraction propagation method is used to iteratively reconstruct the image, so as to achieve simultaneous recovery and reconstruction of the two parameters of amplitude and phase.

2. The terahertz phase recovery imaging method based on unequal spacing measurement according to claim 1, characterized in that: In a single iteration step, The sampling interval sequence from the initial sampling interval position to the farthest sampling interval position is {Z}={Z0, ΔZ i-1 }; where Z0 is the initial sampling interval and is set to a fixed value, ΔZ i-1 is a natural number of unequal sampling intervals, i=2…n+1; The intensity sequence image {I} is a sequence image of intensity values ​​at sampling points corresponding to each sampling interval, {I} = {I0, I i-1 }, where I0 is the intensity value of the initial sampling interval point, I i-1 is the measured intensity image of points with unequal sampling intervals, i=2...n+1 is a natural number.

3. A terahertz phase recovery imaging method based on unequal spacing measurement according to claim 1 or 2, characterized in that: Unequal sampling interval ΔZ i-1 The setting is based on the Fresnel number difference ΔF between adjacent diffraction recording patterns. # value to set.

4. The terahertz phase recovery imaging method based on unequal spacing measurement according to claim 1, characterized in that: The above ii. for a series of intensity sequence images, the Rayleigh-Sommerfeld diffraction propagation method is used to reconstruct the complex amplitude images measured at unequal intervals, including: a). At the initial sampling interval Z0, the initial phase value φ(0) is randomly selected, then the complex amplitude of the initial object surface is b) Starting from n=2, use the Rayleigh-Sommerfeld diffraction formula to sequentially divide U0 by different sampling intervals ΔZ i-1 The forward diffraction propagates to the next acquisition position, and then travels back in the opposite direction to the position at the initial sampling interval Z0 to obtain the complex amplitude image of the first position; c) Use the Rayleigh-Sommerfeld formula to reversely diffract the complex amplitude image of the first position after the iteration is completed. The propagation distance Z0 is obtained to obtain a clear complex amplitude image of the object under test in this cyclic iteration of unequal sampling intervals.

5. The terahertz phase recovery imaging method based on unequal spacing measurement according to claim 4, characterized in that: In the forward propagation and reverse return process in step b), the measured intensity values ​​I at each unequal sampling interval position are i-1 Update the amplitude by the square root of , and keep the phase value calculated in the previous step unchanged, so as to obtain the new U in (x, y) is used as the complex amplitude of the input plane for the next step of propagation calculation, and the propagation of the phase complex amplitude image at unequally spaced positions continues.

6. The terahertz compression imaging optimization method based on data selection according to claim 1, characterized in that: The Rayleigh-Sommerfeld diffraction propagation formula is: Where λ is the incident terahertz wavelength, k = 2π / λ is the wave number, j represents the imaginary part of the complex number, z is the distance between adjacent recording surfaces, and U in is the complex amplitude in the input plane, U out is the complex amplitude in the output plane, (x,y) and (f x ,f y ) are spatial domain and frequency domain coordinates, F and F -1 are the forward and inverse Fourier transforms.

7. The terahertz phase recovery imaging method based on unequal spacing measurement according to claim 1, characterized in that: The iteration rounds of the unequal sampling intervals are terminated according to the following iteration termination criteria: In each round, the root mean square error between the intensity image of the complex amplitude restored in each iteration and the intensity recording surface at the first Z0 is calculated: , where t is the iteration round and M is the number of N pixels; When the MSE is lower than the set threshold, the iteration is terminated, and the iterative process of phase recovery for round i is completed.

8. A terahertz phase recovery imaging system based on unequal spacing measurement, characterized in that: include: Terahertz emission source, imaging object, terahertz detector electronic computer and three-dimensional translation stage; The emission source is arranged at the origin of the three-dimensional coordinate system, and the image object and the detector are arranged in sequence in the direction of the z-axis with the terahertz wave emission direction as the z-axis; the emission source emits terahertz waves, and the terahertz detector obtains the intensity value of the imaged object transmitted by the terahertz waves; The three-dimensional displacement stage includes a servo motor, a driver, an x-axis transverse linear motion module, a y-axis longitudinal linear motion module, and a z-axis terahertz wave emission direction motion module. The terahertz detector is fixed on the transverse motion module. The servo motor drives the transverse linear motion module and the longitudinal linear motion module to move, thereby adjusting the position of the terahertz detector in the xy two-dimensional plane. The servo motor drives the z-axis terahertz wave emission direction motion module to change the sampling positions of the various unequal sampling intervals residing on the z-axis. The driver is connected to an electronic computer via a communication interface. The electronic computer comprises a storage unit and a processing unit, the storage unit stores a program, and the processing unit loads the program to execute the steps of the method described in any one of claims 1 to 7.

9. The terahertz compression imaging optimization system based on data selection according to claim 8, characterized in that: The servo motor is used to drive the z-axis terahertz wave emission direction motion module to change the sampling positions of the unequal sampling intervals on the z-axis, including: using the unequal sampling interval sequence {Z} as an adjustment parameter, driving the terahertz detector on the three-dimensional displacement stage from the initial sampling interval position to the farthest sampling interval position, and residing at the sampling positions of the unequal sampling intervals on the z-axis in a forward direction away from the emission source and then in a reverse direction asymptotically approaching.

Images